Furnace thermocouple problem

California · Aug 11, 2004

This old farmhouse has 'central heat' in the form of a freestanding 30k btu heater in the main room.

The heater was new in 1999 and I've put three replacement thermocouples in since then. Tonight it died again. I can't get it to sustain the pilot flame when I release the overrride no matter how long I held the override down.

The pilot flame seems vigorous and is aimed correctly to warm the thermocouple so I don't think that is the problem.

Is this likely a string of bad thermocouples, or is there something in the main valve that can go bad and then ignore the thermocouple? Have I reached the point where it is simpler to replace the entire heater?

Any comments will be very much appreciated.

hwp · Aug 11, 2004

Sounds like a typical thermocouple situation to me. In my previous house I had a furnace with that kind of system and I had to replace the thermocouple every couple of years. Now I have electronic ignition so don't have that problem. Just keep a spare thermocouple on hand so that you can change it when needed.

Egon · Aug 11, 2004

Thermo couples arent that expensive and pretty easy to change. Main thing is the shutoff valve closes when the thermocouple fails.

As for replacement of the heater that's a difficult question to even attemp to answer due to all the variables involved.

Egon

Birdhunter1 · Aug 11, 2004

A new heater is much more expensive than a few thermocouples. I use to be an HVAC servie tech/installer and one night I went to a house and found a bad thermocouple, when I went to my service truck I found that the other service man had got the only therm thermocouple long enough for this job off of my truck that morning. Rather than drive all the way back to the shop I took sandpaper and cleaned the snot out of the flame snesor and stuck it back in. Miraculously it worked but I knew it would only be a temp. fix, I showed the homeowner what I had done and since it was a mild night I told him I would bring a new one by the next day. The next day on the way home from work I gae it to him and he said he would put it on, I saw him a week later and he said he forgot to do it that night and it was still working. I asume you hae checked that the sensor is in the center of the flame and not the end.

Dozernut · Aug 11, 2004

A few small things you might check, make sure the copper wire is not touching anything that would act as a heat sink. Check for corrosion on your thermocouple connection. Maybe you need to bend the mounting plate so the flame engulfs the sensor better. See if the gas jet for the pilot is partially plugged.
I always buy at least two spare thermocouples because when the furnace goes out it is most likely 2:00 am and below zero.

SPIKER · Aug 11, 2004

ok this is a typical problem and one thing that EVRYONE has missed so far it the FURNACE GROUND! you HAVE to have a good ground to the unit or no matter how many NEW thermocouples you put on they will only work for a little while and quit... been there done that many times

like others said the thermocouple it's self usually will not go BAD persey, but they do carbon up and then they get changed (breaks off all the carbon) and it works again (it's mirical it was fixed HAD to have been a bad TC) ok anyhow use a bit of sand papoer to CLEAN off the junction of all the parts, and any rust , make sure there is a GOOD gorund wire running form the Furnace to the G in the home wire panel and then make sure the gas line is grounded as well. (Use a wire loope around the gas valve to catch BOTH sides of the piping and insulate the valve form acting as a ground path.

if these do not work, then you probably are getting a valve that is going south, there is small (~5 millie amp dc) signal reader inside which keeps the valve OPEN when the TC reads the flame, (flame generates the electrical current by heat/proxsimity to the TC and if the TC is not mounted correctly then it bleeds off the voltage, also DON"T KINK THEM, they center bi-metalic tube must not be kinked. it will break the insulation and the current will/can be lost through the mount and any metal that the external metal tube touches.) ok

HOPE THA HELPS

Martk M /forums/images/graemlins/smile.gif

California · Aug 31, 2004

OK, I tried everything suggested above.

Except for the electrical stuff. This unit is too simple to have a millivolt thermostat, rather it's equivalent to an old water heater. The thermostat for the main burner is just a mechanical sensor tube hanging off the valve unit.

And... my problem is unrelated to the main burner thermostat. The problem is that the pilot flame won't stay lit when I let go of the override button.

It took several attempts, about 10 minutes total, to get the pilot flame to hold after installing the new thermocouple. This is the fourth thermocouple in four years. There's plenty of flame on it. As soon as I let go of the override button, the flame dies instantly but the thermocouple is so hot it continues to glow red/orange for a full 10 seconds before it cools down to dark and invisible in the burner chamber. I think it is sufficiently warmed.

A new aspect: I discovered a pair of wires coming from the thermocouple area of the valve and going to a sensor bolted onto the chimney just as it leaves the main burner chamber. I found the original installer's manual and the parts list hints that this is a cold-chimney sensor. So it looks like I need to override or replace this sensor as the next step in diagnosing why this thing is so hard to light.

Has someone with more experience than I replaced a cold-chimney sensor? Again, any advice?

Thanks!

SPIKER · Aug 31, 2004

one other thing, there MAY be some carrosion built up down in the thermocouple mount to the valve, the small plunger looking thing has to contact a small plate down inside the valve this is where the milliamp current goes and this locks a small solinoid OPEN so the valve stays ON. the only other cause should/could be that the valve it;s self is starting to GO BAD, as in requiring more and more current to stay open.
I'm not familiar with teh cold chimminy switch. it is worth a try to see if it helps..

MarkM

gspig · Aug 31, 2004

Are you on LP or natural gas. I am on LP and have similar problems every winter. I suspect in my case that there is a gas supply problem. Once the heater decides to stay lit, I don't have a problem until I turn it off for the season. I think that there may be a drop in supply pressure or an "air bubble" in the line.

Junkman · Aug 31, 2004

At this point, I suggest that you call a contractor that is experienced with repairing these things. I would hate to read about your home going bang in the middle of the night because of some malfunction. Natural and propane gas is not something that you want to mess with when there is a problem that is difficult to correct. If you bypass the wrong item, then it could possibly blow up with you in front of it. I know that we all think that we can fix almost anything ourselves, but this is one time that I believe that you should get a experience person in to look at it. If you mess up your car or tractor it only stops working. If you mess up the gas valve, your life is a risk, and your families also.... Just my unsolicited opinion from experience working with gas appliances. Mine blew up a few hours after I cleaned and reset the thermocouple and we had heat again. It was a defective gas valve that I didn't know about. It was on a recall for just this problem. Luck for us, the heater was an outside slab mounted unit. The only thing that was wrecked was the heater and some duct work...

California · Sep 3, 2004

Problem solved!

I'm documenting this here in case it might help someone else in the future.

This 1999 heater has a safety device I have never seen before, and since no one else described it I assume this is a new feature.

A sensor named 'Vent Safety Spill Switch' is located on the back of the heater below the chimney connection. Per the installer manual: "The switch, when activated, will extinguish the pilot flame."

I still don't understand if it senses heat or some combustion gasses, but according to the manual it activates if the chimney is blocked, too short, or isn't type B vent pipe. Or if the vent has too many bends or is too close to horizontal.

The safety switch has blade terminals and I had already crimped the wiring harness's connectors tighter to assure a good connection. Finally standing on my head between the heater and the wall I found another set of blade connectors halfway between the safety switch and the gas valve in the heater. They were loose, I crimped the connectors to make them slide snugly on to the blades, and problem solved.

To verify that thermocouples weren't the problem I reinstalled the one I took out last week, and then the one that was in it prior to that. All three thermocouples worked properly, about 45 seconds of holding the override was all it took to light the pilot. I don't think before this repair we ever got it lit in under five minutes, since it was new in 1999.

Thanks to everyone for your comments! They inspired me to keep analysing this mystery. I was ready to replace the heater's main gas valve assembly which as it turned out, wouldn't have fixed anything.

And Junkman, thanks for your concern. That inspired me to check for recalls. So far as I can find there isn't one for this heater.

Egon · Sep 3, 2004

Good to hear the " Thermocouple Mystery " is solved.

Never heard of the safety device you mentioned. Is your heater one of the high efficiency modelds that vent through the wall with a plastic pipe?

Egon

California · Sep 3, 2004

Ergon,

I doubt this heater is a premium high efficiency unit. It specifies ordinary Type B vent pipe. (double wall).

This link has photos showing the front and back of the heater. The 'Vent Safety Spill Switch' is below the vent on the back. It is mounted on a spacer to hold it a half inch away from the cabinet.

I had assumed that was an overheating shutdown safety sensor. After reading their description, I think now it may sense if combustion gasses run backward and spill out of the chamber's intake.

The sensor's wires connect to the gas valve in an unusual way. There is a wide saw-cut straight down through the fitting where the thermocouple screws in. The sensor wires enter this slot and terminate at a (replaceable) button that lies between the tip of the thermocouple and the bottom of its socket.

'Wiring Harness w/ Bluebird' is the description in the parts list for this button and its wires.

I've never seen anything like it!

Egon · Sep 3, 2004

Thanks for the information.

Resettable Vent Spill Safety Device automatically shuts unit off in the event of flue blockage or incorrect vent installation.

Never seen one but now know about it. Glad you were able to figure it out and let us know.

Egon /forums/images/graemlins/grin.gif

Oilman74 · Dec 26, 2021

Jump out the two wires that lead up to the flu before the chimney that’s in over fill switch or when the draft is blocked. Or jump it out with a jumper wire where it goes into the gas valve . it’s extra safety but you really don’t need it .

Hay Dude · Dec 26, 2021

Birdhunter1 said:
A new heater is much more expensive than a few thermocouples. I use to be an HVAC servie tech/installer and one night I went to a house and found a bad thermocouple, when I went to my service truck I found that the other service man had got the only therm thermocouple long enough for this job off of my truck that morning. Rather than drive all the way back to the shop I took sandpaper and cleaned the snot out of the flame snesor and stuck it back in. Miraculously it worked but I knew it would only be a temp. fix, I showed the homeowner what I had done and since it was a mild night I told him I would bring a new one by the next day. The next day on the way home from work I gae it to him and he said he would put it on, I saw him a week later and he said he forgot to do it that night and it was still working. I asume you hae checked that the sensor is in the center of the flame and not the end.

That’s all I do and it works great.

MossRoad · Dec 26, 2021

17 year old zombie thread revival.

Welcome to TBN.

California · Feb 26, 2022

Oilman74 said:
Jump out the two wires that lead up to the flu before the chimney that’s in over fill switch or when the draft is blocked. Or jump it out with a jumper wire where it goes into the gas valve . it’s extra safety but you really don’t need it .

I just noticed this prehistoric thread has recent posts.

Update. Things I've learned since 2004. Maybe this will help someone else encountering this problem:

* After that 'Resettable Vent Spill Safety Device' on the back of the heater tripped a few times, it stayed tripped for hours, then permanent, showing infinite resistance from one terminal to the other. A replacement wasn't available at the local HVAC supply house so I found it online. (I found they are rated in degrees. So it senses temperature, not gasses).

* I used a common automotive blade fuse as a jumper in place of the safety device, until I received the replacement.

* I finally called a HVAC pro to troubleshoot why the heater was tripping its safety device. He said the chimney (Type B vent pipe) didn't extend far enough above the eaves to draw properly. He extended the height with an offset to get above the roof line. Problem solved. It ran for a decade without problems. Then it quit, but cleaning all the blade connectors between the safety device and the gas regulator restored normal function. The heater has now been in use 20+ years and continues to work fine.

MossRoad · Feb 26, 2022

Thanks for the update.

Just to make you feel better, you were 17+ years younger when you posted that originally.

kenmac · Feb 27, 2022

California said:
I just noticed this prehistoric thread has recent posts.

Update. Things I've learned since 2004. Maybe this will help someone else encountering this problem:

* After that 'Resettable Vent Spill Safety Device' on the back of the heater tripped a few times, it stayed tripped for hours, then permanent, showing infinite resistance from one terminal to the other. A replacement wasn't available at the local HVAC supply house so I found it online.

* I used a common automotive blade fuse as a jumper in place of the safety device, until I received the replacement.

* I finally called a HVAC pro to troubleshoot why the heater was tripping its safety device. He said the chimney (Type B vent pipe) didn't extend far enough above the eaves to draw properly. He extended the height with another section of Type B. Problem solved. It ran for a decade without problems. Then it quit, but cleaning all the blade connectors between the safety device and the gas regulator restored normal function. The heater has now been in use 20+ years and continues to work fine.

must be on a natural draft furnace, and not a vent motor assist type. Does the B vent run up through the interior of the house and exit the roof, or does it go through an outside wall, and run up the outside wall of the home ?

Reason I ask about how the B vent is run is, when it's cold out, sometimes it takes a little bit to get the B vent warm enough to establish a draft. Or, if you have more lateral run vs vertical height, you can have issues.

What's funny is, I have seen vents terminate just on an outside wall, and thought not code, vent just fine